Porous polymeric matrices comprising particles have been described before.
For instance in U.S. Pat. No. 6,048,457 cast porous polysulfone membrane structures comprising sorptive particles such as active carbon, (fumed or derivatised) silica or (functionalised) polystyrenedivinylbenzene beads are described. It concerns cast-in-place structures confined in pipette tips for small scale sample preparation.
Another example is U.S. Pat. No. 5,258,149 in which a hollow fibre membrane comprising polysulfone polymer and silica is described. It is stated that silica acts as a pore former and viscosifier in membrane formation and that fibres with silica are not microporous until the bulk of the silica is removed by treatment with base. The hollow fibre membrane is immobilised by heat treatment under pressure in the presence of polyacrylic acid. The polyacrylic acid binds to the fibre walls and acts as an affinity agent for low density lipoprotein cholesterol complex (LDL-C).
In U.S. Pat. No. 5,238,735 the preparation of a microporous polyolefin hollow fibre comprising synthetic resin particles is described by extruding a mixture of (co)polyolefin(s), synthetic resin particles and plasticiser, from a melt at a temperature of 230° C., into a strand which was cut into pellets. The resulting pellets were extruded from the melt at a temperature of 215° C. through a hollow fibre producing nozzle. In order to introduce the desired porosity the unstretched hollow fibre is monoaxially stretched by a roll-stretching method resulting in a molecularly oriented microporous hollow fibre.
Also in WO 00/02638 a porous polymeric matrix comprising substantially immobilised material is described. Such a flexible sheet membrane (flat, pleated or rippled) has a selectively permeable skin on the outer surface. In particular the preparation of a membrane by flow casting a slurry-like blend of polyurethane and activated charcoal onto a polyester support is described. It is mentioned that the blend can also be extruded onto a support. Further it is also noted that the membrane can be made without an integral support, for instance by applying the blend to a drum and thereafter peeling the membrane off the surface of the drum. In passing it is noted that also other configurations than flat sheet membranes can be formed such as fibres, rods and tubes. However, besides the embodiment of flow casting a membrane onto a support none of the other suggestions are enabling disclosed.
In WO 98/34977 a porous composite product formed from at least one water-insoluble polymer, at least one water-soluble polymer and at least 20% of at least one filler material, in particular active carbon, is described. The product is obtained by a melt extrusion process, using an extruder. The porosity in the product is introduced by eliminating the soluble polymer from the extruded product. It is stated the polymeric material is non-fibrous and rather concerns a film of porous composite products.
Thus, in the art methods are known to prepare porous polymeric material comprising particulate material in one step from an appropriate mixture of starting components. Such a material is prepared by a casting process and either is limited in its three dimensional size by the housing it is cast into or is in the form of a sheet. Such casting processes are not suitable for the preparation of fibres.
In order to prepare porous polymeric fibres comprising particulate material an additional process step is required to introduce the desired porosity. After the step of preparing the fibre comprising particulate material either particulate material is removed form the non-porous fibre or the non-porous fibre is stretched resulting in porous fibres. Only in the latter case a microporous fibre comprising particles having a certain (sorptive) function is obtained.
Disadvantages of the known porous polymeric fibre preparation processes are that they involve additional process steps after the formation of the fibre to come to a final product. It is desirable to have a more efficient preparation process. Depending on the actual process steps that need to be taken to come to the final product suitable starting materials have to be selected with properties that can sustain the conditions of the additional process steps. Obviously such a requirement puts limitations on the polymeric material that can be used. Furthermore it puts limitations on the type of particulate material that can be comprised in the polymeric matrix. A high degree of particle loading will reduce the mechanical strength of the fibre and therefore restrict the stretching procedure. The degree of loading will be limited by the force required to reach sufficient stretching of the matrix material. By stretching of the particle comprising material the particulate material can drop out of the porous structure to be formed. In processes which involve melt extrusion only particulate material that can sustain temperatures required to melt the matrix polymer can be applied. It is not uncommon that these temperatures are well above 200° C.
DD A 233,385 discloses a method for the preparation of porous fibres, comprising a one-step phase inversion or so-called wet-spinning process. Immediately after extrusion the fibre enters a coagulation bath. Particles are applied to maintain porosity during drying at elevated temperatures; the accessibility and functionality of the particles are less critical therein. It is stated that the properties and behaviour of the end-product are essentially determined by the chemical structure of the polymer used.
Drawback of a method according to DD A 233,385 is that direct spinning in a coagulation bath with less than 60 wt. % solvent results in rather dense exterior surfaces and limited particle accessibility. However, an increase in the amount of solvent results in difficulties of controlling the spinning process; due to delayed demixing of the nascent fibre solidification takes too long.